Converters converting direct current to direct current are often used as an input stage of an inverter, for example. They are referred to below as DC/DC converters. They may be designed as a boost converter, a buck converter or a combined buck/boost converter. At least one clocked switching member is arranged in a power circuit of the DC/DC converter. Depending on the switching state of the switching member, electrical energy is converted into magnetic energy in the inductor and intermediately stored therein or said intermediately stored magnetic energy is converted back into electrical energy and output again by the DC/DC converter. By way of example, MOSFETs (metal-oxide semiconductor field-effect transistors), JFETs (junction gate field-effect transistors) or IGBTs (insulated gate bipolar transistors) or other transistors may be used as switching members.
In the case of a DC/DC converter, the switching member is usually clocked using a pulse-width-modulated (PWM) signal. Different operating modes of DC/DC converters are known, which operating modes differ in switching frequency and/or switching points in time respectively. Known operating modes may be the so-called CCM mode (continuous conduction mode), the DCM mode (discontinuous conduction mode), the boundary conduction mode (BCM) or the RPM method (resonant pole mode). An overview of various operating modes of voltage converters can be found in the document “Highly Efficient Inverter Architectures for Three-Phase Grid Connection of Photovoltaic Generators”, K. Rigbers, Shaker Verlag, Aachen, 2011.
The amount of energy that can be intermediately stored in the inductor during one switching cycle is determined by the inductance value of said inductor at maximum magnetization. In order to achieve an inductance value which is as high as possible and at the same time of small installation size and with a low number of windings, inductors comprising a core composed of material with a high magnetic permeability are usually used. The core advantageously increases the inductance value only up to a saturation magnetization, the level of which depends on the selected core material. If the inductor with the core is operated in saturation, only the leakage inductance is still available for further energy absorption.
In the case of DC/DC converters which are operated unidirectionally and, correspondingly, in which energy only flows in one predefined direction during operation, for example from an input side to an output side, the core of the inductor is only magnetized in one magnetic direction during operation. In this case it is known to pre-magnetize the core of the inductor in the opposite magnetic direction, for example by using permanent magnets within the core. Usually the pre-magnetization doubles the magnetization range which can be used during operation of the inductor. However, for an opposite energy flow direction in the case of such a DC/DC converter, only a very small magnetization range would be available to the inductor until the core reaches its saturation magnetization. Moreover, only the low leakage inductance could additionally be used in this energy flow direction. Therefore DC/DC converters with pre-magnetization of the core of their inductors have only been used unidirectionally up to now.
Inverters of a photovoltaic installation are used to convert direct current supplied by a photovoltaic generator into alternating current for feeding into a power supply grid. Often there is the demand to also supply reactive power to the power supply grid. We could call the energy flow direction when feeding active power into the grid the “main energy flow direction”. So in order to provide reactive power to the grid the DC/DC converter must be configured for a bidirectional operation in order to operate in a direction opposite to the main energy flow direction. This is especially the case if the DC-link is located—with respect to the main energy flow direction—upstream of the DC/DC converter or if the DC/DC converter also has the functionality of modelling the current shape.